Utilization of ammonia as a nitrogen source in the silkworm, Bombyx mori

This study evaluated the relative effectiveness of ammonium salt in comparison to non-essential amino acids for growth of the silkworm larvae, Bombyx mori. Newly ecdysed 5th-instar larvae were fed chemically defined diets containing only the ten essential amino acids supplemented with diammonium citrate (DAC), potassium glutamate (K-Glu) or a mixture of acidic plus non-essential amino acids (NEA). Alterations in larval growth and the haemolymph components as a result of the supplemented diet were used to investigate the utilization of nitrogen sources by this species. Promotion of growth of larvae and silk glands was achieved by the addition of DAC or K-Glu, but NEA was most effective. Haemolymph protein content increased with increased levels of nitrogen in the supplemented diet. The composition of free amino acids in the haemolymph was altered as the level of DAC or K-Glu in the diet increased. Among the essential amino acids, valine, methionine, isoleucine, leucine and phenylalanine decreased, whereas almost all non-essential amino acids except for tyrosine and ornithine greatly increased. To confirm that dietary ammonia was used as a nitrogen source, 5th-instar larvae were fed a diet containing 15N ammonium acetate. Mass spectroscopic analysis revealed that a large amount of 15N was incorporated into larval protein fractions, especially the silk glands. These results indicate that dietary ammonium nitrogen is converted by silkworm larvae into amino acid nitrogen and utilized for protein synthesis in the same manner as non-essential amino acids.